The present invention relates to an improved method of securing a surgical scalpel blade and a handle one to the other and particularly relates to a method for cold staking the blade to the handle.
Disposable surgical scalpels typically consist of a stainless steel or carbon steel blade and a separately manufactured molded plastic handle. Consequently, a means for attaching the blade and handle one to the other is required. Conventionally, the handle is molded to provide at one end a seat for interlocking engagement with the outer profile of the tang of the blade. It is also provided with a key which is received in a keyway formed on the blade. Fundamentally, the attachment must be secure enough to transmit anticipated cutting forced from the blade to the handle and sufficiently tight to prevent relative movement between the blade and the instrument.
In conventional disposable surgical scalpels, the key on the handle is made to fit the entire opening in the blade tang. The key extends beyond the outer surface of the blade and is deformed to displace material over the edges of the blade tang keyhole by application of a tool which reduces the height of the key. Two methods of staking are currently used to accomplish this deformation. One method is hot staking which involves use of a heated tool applied to the top of the handle key. The heat transferred from the tool to the plastic softens it and pressure is used to cause the softened plastic to flow out over the blade surface surrounding the keyway, thereby capturing the blade. Problems associated with this method include process control, non-uniformity of final appearance and strength of the overall assembly.
Ultrasonic staking uses a tool similar to that used in hot staking. Rather than being heated, the tool is caused to vibrate at high frequency, i.e., 20,000-40,000 cycles per second. Upon contact with the handle key, the ultrasonic vibration excites the plastic molecules, causing the plastic to soften and flow. The degree of success with respect to ultrasonic staking is a function, however, of the different plastic materials used. Most materials of choice for the manufacture of scalpel handles are only marginal candidates for ultrasonic staking and numerous compromises have to be made to use this method of manufacture.
Recognizing the deficiencies of these prior art techniques, a novel and improved attachment between the scalpel blade and the scalpel handle has been formulated in accordance with the present invention. Starting first with the specification of the material of the handle, criterion of cost, autoclavibility solvent resistance, stiffness and dimensional stability indicate that a filled polypropylene would be optimum. While other manufacturers of disposable scalpels have used polypropylene filled with about 20% talc, it has been found that handles with that material are dimensionally less stable and more flexible than desired. A talc filler of 40% by weight, however, has been found to be too brittle. As part of the present invention, a talc filling for the polypropylene handle material of about 25-35% by weight ultimately produces a handle which can advantageously be cold staked and does not have the disadvantages noted above.
It is also necessary to provide a simple economical strong attachment which lends itself to automatic assembly and produces an aesthetically pleasing appearance. Cold staking is employed in the present invention to accomplish those goals. However, tests conducted with cold staking of full profile handle keys tended to split the blade during manufacture. That is, the plastic material flow completely filled the blade keyhole and, as the stake was completed, split the blade. It was found, after further testing, that the strength of the attachment between the scalpel handle and scalpel blade depended on the ends of the resulting stake rather than in the mid-portion of the stake. The result, therefore, is the adoption of a configuration for a handle key which enabled cold staking of high strength and aeasthetic appearance without splitting the blade. It has also been found that, with such new handle key configuration and concomitant cold staking thereof, superior blade retention and strength under adverse loading conditions are achieved. More particularly, the configuration of the handle key which produces these results provides a mid-portion height of the handle key substantially no greater than the thickness of the blade while the end portions of the handle key have a substantially greater thickness than the thickness of the blade. In this manner, during cold staking in accordance with the present invention, two separate cold staked heads are formed over the blade keyhole. That is, the heads at the opposite ends of the key are staked, whereas the intermediate portion of the key between the end stakes are left unstaked. Forces that would otherwise split the blade are eliminated and the portions of the handle key which carry the greatest load are staked.
In accordance with a particular embodiment of the present invention, there is provided a novel and improved method of securing a surgical scalpel blade and a handle one to the other comprising the steps of providing a surgical scalpel blade with a through keyway and forming a handle having a surface for receiving a side face of the blade adjacent an end of the scalpel handle and a key projecting from the surface. The key has a pair of spaced elements projecting from the surface distances greater than the thickness of the blade and at least one additional element between the spaced elements projecting from the surface a distance substantially no greater than the thickness of the blade. The blade is disposed on the handle with the key received in the keyway and the side face of the blade bearing against the handle surface. The spaced elements are cold staked to overlie portions of the opposite side face of the blade, thereby to secure the blade and handle one to the other. Preferably, the method hereof includes cold staking only the two elements, leaving in final securement the one element between the two elements unstaked. Additionally, the method hereof includes forming the handle of a polypropylene material with a filler, preferably talc, of about 25-35% by weight of the handle material.
Accordingly, it is a primary object of the present invention to provide a novel and improved method for securing a surgical scalpel blade and handle one to the other wherein cold staking is employed in a particular manner to produce a joint of high strength and aesthetically pleasing appearance without blade splitting during manufacture.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.